1. Field of Invention
This invention pertains to ballistic armor. More particularly, this invention pertains to ballistic armor formed from polymer encapsulated glass and polymer encapsulated ceramic materials.
2. Description of the Related Art
In designing ballistic armor, desired armor protection levels can usually be obtained if weight is not a consideration. In many armor applications other than personal armor, weight is not a critical factor, and thus traditional materials, such as steel, can offer some level of protection from ballistic projectiles and shell fragments. Steel armors also offer the advantage of low cost and can serve as structural members of the equipment into which they are incorporated.
However, in many other armor applications, there is a premium put on armor weight. Some areas of application where lightweight armor are desirable include ground combat and tactical vehicles, portable hardened shelters, helicopters, and various other aircraft used by the Army and the other military services. Another example of an armor application in need of reduced weight is personnel body armor worn by soldiers and law enforcement personnel.
In recent decades, certain hard ceramic materials have been developed for certain armor applications. These ceramic-based armors, such as alumina, boron carbide, silicon carbide, and titanium diboride ceramics provide the advantage of being lighter in mass than steel and provide ballistic stopping power comparable to steel. Thus, in applications in which having an armor design with the lowest possible weight is important, low specific gravity armor materials are highly desirable.
Ballistic ceramics are extraordinarily hard, strong in compression, and relatively light weight, making them efficient at eroding and shattering armor-piercing threats. However, ballistic ceramics often experience brittle fracture due to excessive tensile stresses on the back face of the armor body. After one impact of sufficient energy, a previously monolithic ceramic fractures extensively, leaving many smaller pieces and a reduced ability to protect against subsequent hits in the same vicinity. By reducing tensile stress within the ceramic armor body, the kinetic energy of the projectile can be absorbed completely within the projectile, e.g., ideally complete self-destruction at the surface of the armor body, or more typically, shattering of the ceramic while the projectile undergoes self-destruction as its kinetic energy is depleted to zero.
Conventional ceramic armor materials typically employ a laminated structure comprising a layer of ceramic material such as boron carbide and a layer of reinforced fabric such as Kevlar®. The ceramic layer typically faces the expected incoming projectiles and is typically covered with what is called a spall shield—a thin, flexible layer which is provided as the outer layer facing the incoming projectiles. This layer is typically either rubberized, or is constructed of ballistic nylon cloth, felt, or resin-impregnated glass fabric. The spall shield is designed to prevent ejection of high velocity fragments of ceramic or projectile particles subsequent to the impact by the projectile.
In the laboratory, ceramics show much higher performance in ballistic armor applications when their boundaries are heavily confined. The two key parameters are suppression of cracked tile expansion and putting the ceramic in an initial state of high compressive stress to delay or stop it from going into a state of tensile stress during impact. If the ceramic tile is not encased, the fractured pieces can easily move away from the locale of the impact, and residual protection is lost.
Snedeker et al., use a hybrid metal/ceramic approach in U.S. Pat. No. 5,686,689. Ceramic tiles are placed into individual cells of a metallic frame consisting of a backing plate and thin surrounding walls. A metallic cover is then welded over each cell, encasing the ceramic tiles. In U.S. Pat. No. 6,601,497, Ghiorse et al., describes wrapping a band of metal material around the perimeter of a ceramic tile so as to place the tile in a compressive state. Also, U.S. Pat. No. 4,739,690, issued to Moskowitz, teaches a spall shield for an armor plate wherein the spall shield contains an outer layer of plasticized resin.